Due to the inevitable depletion and negative environmental impact of fossil fuels on the environment, increasing attention has been focused on converting feedstocks into renewable biofuels. Lignocellulosic biomass is a feedstock of particular interest. During the conversion process, pretreatment provides the key to unlocking the protective structures in biological feedstocks so that enzymatic hydrolysis of the carbohydrate fraction to monosugars can be achieved more rapidly and with greater yield (Wyman, 2007; Yang and Wyman, 2008). Cellulose in lignocellulosic biomass exists in the form of fiber, macrofibril, and microfibril which are surrounded by lignin and hemicellulose (Somerville et al., 2004). Some researchers attribute the recalcitrance of lignocellulosic biomass to two main root causes: 1) the presence of the lignin seal and hemicellulose links on the surface of cellulose which prevent cellulase from accessing the substrate, and 2) low accessibility of crystalline cellulose fibers, which restricts cellulase from working efficiently (Zhang et al., 2007). During pretreatment the matrix is broken to expose cellulose fibers for enzymatic attack.
In cellulosic ethanol production, processing accounts for up to 67% of the total cost with pretreatment being the single most expensive unit operation (Wyman, 2007). Researchers are scrutinizing each step in biomass conversion to minimize biofuel production costs. Similar to its successful counterpart in the petrochemical industry, the ultimate goal of a biorefinery is the efficient fractionation of lignocellulosic biomass into multiple product streams that contain value-added compounds so that the overall economics of a biofuel production facility can be significantly improved.
Numerous pretreatment concepts have been proposed and tested over the years. Steam treatment, or steam explosion, with and without the addition of a catalyst is one of the oldest methods that has been investigated for the pretreatment of a variety of lignocellulosic biomass feedstocks for the production of fuel ethanol (Soderstrom et al., 2004; Palmarola-Adrados et al., 2004; Sassner et al., 2005; Varga et al., 2004). Other treatment methods studied include supercritical CO2 (Kim and Hong, 2001), supercritical water (Nakata et al., 2006), wet oxidation (Palonen et al., 2004), alkaline peroxide (Sun et al., 2000; Saha and Cotta, 2006), and hypochlorite treatment under acidic conditions (Hromadkova and Ebringerova, 1995).
A coordinated study supported by a USDA Initiative for Future Agricultural and Food Systems grant has examined the performance of promising biomass pretreatment methods. Using a single feedstock, common analytical protocols and consistent data interpretation, five research teams documented the technical and economical feasibility of six pretreatment techniques (Wyman et al., 2005; Eggeman and Elander, 2005). Almost all six of the pretreatment methods, including dilute acid (Lloyd and Wyman, 2005), flow through (Liu and Wyman, 2005), hot water (neutral pH) (Mosier et al., 2005a), ammonia fiber/freeze explosion (AFEX) (Teymouri et al., 2005), ammonia recycle percolation (ARP) (Kim and Lee, 2005), and lime (Kim and Holtzapple, 2005) are capable of producing high sugar yield. However, it was found that low cost pretreatment reactors are often counterbalanced by the high cost associated with either pretreatment catalyst recovery or ethanol recovery (Eggeman and Elander, 2005). Moreover, pretreatments at high temperatures, especially those treatments using acid or water, may cause degradation of valuable cellulose, hemicellulose and lignin. They can also result in the formation of inhibitory products that are toxic to microorganisms used for subsequent fermentation of the recovered sugar (Zaldivar and Ingram, 1999; Oliva et al., 2003).
Until now, all of the existing and newly developed biomass deconstruction methods have required harsh treatment conditions, extensive treatment times, high processing costs, or high energy consumption. As a result, it is very difficult to lower both the capital investment and production costs to produce biofuels with a price that is comparable to or lower than starch and petroleum-based fuels. As such, there is a tremendous need to develop improved methods for biomass deconstruction.